Blow molding is a manufacturing process used to produce hollow articles from thermoplastics. Various methods of blow molding hollow articles are known. Typically, the blow molding process begins with melting a thermoplastic material and forming a parison of the thermoplastic. The parison is then clamped into a mold and air or a gaseous medium is pumped into the parison by the use of nozzles that have been inserted into the parison. The gaseous pressure pushes the thermoplastic material outward to match the shape of the mold. Once the thermoplastic material has cooled and hardened, the mold is opened up and the article is ejected.
Various methods are known for producing cellular thermoplastic articles which are generally referred to as “foam” or “foamed thermoplastic” articles. Methods for producing cellular thermoplastic articles include, for example, the use of foaming agents that release gases that expand the thermoplastic materials at their normal processing temperatures; the use of thermoplastic materials containing liquids or solids, or both, that can be removed by extraction or dissolution; and the technique of stretching thermoplastic films containing liquids or solids, or both, to produce interfacial voids, followed by extraction or dissolution. However, the existing processes of making foamed hollow articles suffer from one common problem, which is that the foamed parisons do not have sufficient wall strength. In addition, the methods that depend on extraction or dissolution require the formation of an interconnecting network of pores that allows removal of the dissolved liquids or solids.
When molding cellular thermoplastic articles, it is important to produce uniform cell sizes. Both durability and strength of the molded articles are dependent upon cell size and uniformity. In the blow molding of cellular articles, the usual manner of controlling cell size and uniformity include altering the foaming agent, pressure and temperature of extrusion, and changes to the mixing portion of the extruder. In spite of the various efforts, a need still exists for a process to produce cellular thermoplastic articles containing pores or cells of uniform distribution.
The object of the present invention is to meet the above defined needs and provide further related advantages.